This invention relates to the protection of electrical apparatus and in particular it relates to a method and apparatus for the protection of polyphase apparatus such as electric motors and transformers.
Protection schemes as first developed were analog in nature and preferably included a separate protection for each phase. These schemes generally provided protection in one or more of the following areas: (a) overload, (b) phase unbalance, (c) phase loss, and (d) phase reversal.
Subsequently, protection schemes were designed to operate in digital form and in general followed the analog design. In other words, there was a digital arrangement for each of the three phases including for each phase an analog-to-digital converter, a memory, and associated circuitry. While the protection provided was suitable, the cost for smaller sizes of equipments was disproportionately high.